you need to edit the uvs of your model (assuming it's a poly model) to layout the mesh as a flat 2d projection. You then assign a shader to it. You then paint your texture map within the map of the uvs and save it as a tif (or a targa) and then assign it in the colour channel of your shader.

You need to find a tutorial that you can read because my explanation isn't going to be enough to get you through this.


Alan